Annabelle the kRaZy cat

I'm pretty sure our cat has lost her mind. Suddenly after being alive 15-16 months she has realized there is this thing that follows her every where she goes. It's like she's saying "OMG--there it is! I must attack it NOW!" She's been a pretty docile cat--even a little bit shy around strangers but now she's just concerned about that thing behind her no matter who is here. She chases it round and round, sits and watches it wave, even stares at it like that will make it go away. Yes....Annabelle has a tail.